The human brain has an important structural difference from those of other mammals: what MIT researchers have discovered
Research from MIT - Massachusetts Institute of Technology just published in Nature sheds new light on our understanding of the human brain.
Following a 2018 study that showed significant differences between the properties of human neurons and those of rat neurons, the new research highlights peculiarities that could partly explain why the human brain has followed such a different evolution than the brains of other mammalian species.
Mammalian brain neurons can receive electrical impulses from thousands of other cells: to allow the propagation of impulses to nerve cells are the so-called ion channels, special protein structures with selective mechanism that allow the passage or not of certain ions inside the cells.
In the 2018 study, researcher Mark Harnett of MIT pointed out that he had identified a clear difference between human and rat brains: in particular, the human brain had a much lower density of ion channels than the other.
The result was, according to the scientists, rather unexpected, because until that moment they tended to consider the density of ion channels as a constant, in different species of mammals.
The new study takes its cue from this acquisition, to better investigate the issue and define whether there are substantial differences between the human brain and that of other mammals. What changes, from one species to another, is the size of neurons - not the number of their possible connections in a given volume of tissue.
As Harnett states, "it appears that the cortex maintains a fixed number of ion channels for all species," and this is so that the energy cost of activating ion channels is roughly the same in all species, large or small.
The study on 10 different mammals
In the new study, researchers compared neurons from as many as 10 different mammalian species to try to track down a design that could explain the issue of ion channel density once and for all.
Brain tissue from mustiolus, gerbil, mouse, guinea pig, rabbit, rat, ferret, marmoset monkey, macaque and human were used. From the smallest of mice to humans, then, passing through mammals of various sizes.
This allowed us to cover a range of cortical thicknesses, as well as different neuronal sizes, within the mammalian kingdom. Confirmation of what was expected was not long in coming: the density of ion channels increases with the size of neurons.
In the very small brain of the mustiol, for example, the density of neurons is higher than that found in the rabbit, which has much larger neurons. But because the rabbit has a higher density of ion channels, the density of channels for the same volume of brain tissue is the same: in the mustiole, in the rabbit, and in all other mammals examined. Except humans.
The larger neurons examined are not matched by any increase in ion channel density. In fact, the density is much lower than the researchers expected, so the human brain represents a marked departure from the results for the other mammalian species examined.
The low density of ion channels may have evolved to save the energy needed to "pump" ions and reserve it for other activities, such as creating more complicated synaptic connections, or making brain activity travel faster.
Dr. Harnett is quite convinced of the energy key: "we believe that humans have found a way to become more energy efficient" at the level of brain activity.The MIT researchers' discovery paves the way for important studies, which could explain much about the particular evolution of the human brain.